Optoelectronic devices interact with radiation and electric current. The interaction can be photoelectric where the device converts incident radiant energy (e.g., in the form of photons) into electrical energy. Alternatively Optoelectronic devices often tend to be high voltage and low current devices. Currently many optoelectronic devices, e.g., thin-film photovoltaic (PV) cells and organic light-emitting diodes (OLEDs) are made by depositing patterns of material on a substrate to form the various device layers, e.g., a bottom electrode, an active layer stack and a top electrode (plus auxiliary layers), of individual devices. In order to reduce the cost of thin film optoelectronic devices, such as solar cells, the use of inexpensive substrate materials is important. Cost goals for competitively priced PV energy products dictate materials costs in the range of $25–$50/m2.
Commodity plastics commonly used in web converting operations cost a small fraction of that; an example is poly(ethylene terephthalate), or PET, which is available at about $0.22/m2 for a 1 mil (25 micron) thickness (2–4 mils would be preferred for mechanical strength). On the other hand, PET cannot withstand temperatures such as are required for many desired thin film process steps, especially those used in the manufacture of CIGS (CuInGaSe2) solar cells, which must be processed at 400–500° C. for times in the range of ½ to 1 hour or more.
Polyimides (PI) are well-known materials also used in web processing (for some kinds of flexible circuit tapes, for example) which can withstand these temperatures. Several research groups have, in fact, used polyimides to make flexible CIGS PV cells. The price of polyimides is much higher than PET, however; currently around $8–$10m2 for 2 mil thickness. This is a very substantial cost, and represents a significant impediment to successful commercialization. Polyimides also have some undesirable properties when used at these temperatures; although they do not decompose, the thermal expansion induces stresses that may be problematic for film adhesion, and water absorption and outgassing in vacuum is an issue. Thin metal foils eliminate concerns about thermal stability or mechanical deformation, but they have other problems. Such foils are not sufficiently smooth as received from commercial sources, and must be polished by means which are not well developed and which add cost. Metals are also more expensive than desired. Estimates for stainless steel range for $4/m2 to over $12/m2. Aluminum is cheaper, but may need to be thicker to have adequate strength for processing.
It would be desirable to use a transfer process in which photovoltaic (PV) devices are fabricated on one substrate optimized for fabrication conditions and transferred to another optimized for the final product. Such transfer process have been proposed and demonstrated already several times, both for PV application and for thin film transistors. For example, D. Rudmann, et al., “Development of Flexible Cu(In,Ga)Se2 Solar Cells on polymers with Lift-Off Processes”; in 16th European Photovoltaic Solar Energy Conference and Exhibition, Glasgow, United Kingdon, 1–5 May 2000, have demonstrated the use of NaCl as a sacrificial layer allowing the detachment of CIGS solar cells from a glass fabrication substrate. R. E. I. Schropp, et al., “Novel Amorphous Silicon Solar Cell Using a Manufacturing Procedure with a Temporary Superstrate”, Materials Research Society Proceedings, vol. 557, p. 713 (1999) describe the fabrication of amorphous silicon solar cells on an aluminum substrate (or “superstrate”), after which a polymer is laminated on top and the metal etched away.
Unfortunately, these prior art processes suffer from several disadvantages. If the metal has to be removed by etching, there is nothing to be gained from the cost viewpoint compared to leaving it in place. (Schropp, et al. used this technique because the design of their device required the light to enter from the side that would have had a metal substrate, while use of a polymer during fabrication was prohibited by temperature requirements). In the case of Rudmann, et al., the glass substrate can be reused, but the removal of the sacrificial layer requires etching of a very thin layer from the side over a large area, which makes it undesirably slow.
Thus, there is a need in the art, for a thin film fabrication transfer process that overcomes the above disadvantages.